(1) Field of the Invention
The invention relates to a THz antenna array comprising a plurality of THz antennae, wherein a THz antenna has a photoconductive region and a first electrode and a second electrode which are arranged spaced apart from one another by a spacer region which extends laterally over at least a part of the photoconductive region. The invention further relates to a method for producing a THz antenna array comprising a plurality of THz antennae, wherein a THz antenna has a photoconductive region and a first electrode and a second electrode which are arranged spaced apart from one another by a spacer region which extends laterally over at least a part of the photoconductive region.
(2) Prior Art
THz antennae can be constructed and manufactured in different ways, it being possible to employ these inter alia as receivers and/or as transmitters.
A first fundamental form of a THz antenna provides a semilarge single antenna structure designed for the range between microscopically small structures (less than 100 μm) and macroscopic millimeter structures (>1 mm). Such a THz antenna is described by Stone et al. in the article “Electrical and Radiation Characteristics of Semilarge Photoconductive Terahertz Emitters” in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 52, No. 10, October 2004.
U.S. Pat. No. 5,401,953 discloses an integrated module for generating radiation in the submillimeter range, the module comprising an array of N photoconductive switches which are biased by a common voltage source and an optical path difference of a common optical pulse providing a repetition rate with different optical delay for each of the switches. The N switches are triggered by a pulse migrating along the entire array of N switches up to a single antenna which as a point source radiates submillimeter radiation spherically in all directions.
In contrast the THz antenna arrays of the type identified at the outset composed of a plurality of THz antennae or THz antenna structures exhibit improved power and modulatability of the same as well as improved directional characteristics. A THz antenna or THz antenna structure fundamentally comprises two electrodes spaced apart with an intervening photoconductive material, i.e. usually a region containing semiconductive material in which charge carriers are optically generable. At the same time the individual THz antennae or THz antenna structures usually have microscopic dimensions. A problem with this is the decoupling of the individual THz antennae as elements of the array in order to prevent destructive interference of the THz distant field—as a rule, e.g. in finger structures, neighbouring elements in the array, e.g. two fingers in each case with intervening photoconductive material, are biased with reciprocal polarity. For this purpose hitherto different possibilities for decoupling the individual elements of the array have been provided.
In the article by Saeedkia et al. “Analyses and Design of a Continuous-Wafer Terahertz Photoconductive Photomixer Array Source”, IEEE TRANSACTIONS ON ANTENNA AND PROPAGATION, Vol. 53, No. 12, December 2005, the possibility of location-dependent modulation of the optical excitation by means of frequency mixing of two lasers is described. The optical intensity modulation achieved by frequency mixing generates charge carriers emitting THz radiation only in those antenna structures or antennae as elements of the array in which the charge carriers are subject to an electric field in the same direction. This ensures constructive interference in the THz distant field. This, however, presupposes that the optical excitation modulation is adapted as accurately as possible to the arrangement of the THz antennae in the THz antenna array. For this reason this method proves to be comparatively inflexible, costly and susceptible to error. Moreover, additional components for frequency mixing are needed. The same applies to approaches which use the generation of a binary grid for excitation modulation.
In the article by Dreyhaupt et al. “High-intensity terahertz radiation from a microstructured large-area photoconductor” in APPLIED PHYSICS LETTERS 86, 121114 (2005), this disadvantage is eliminated in that the optical excitation in certain regions between the THz antennae in a THz antenna array is suppressed by optically absorbent materials. In this case THz-emitting charge carriers can be generated optically only in those regions of the THz antenna array in which they are subject to an electric field in the same direction. The photoconductive material generally present between all neighbouring electrodes—the substrate usually—is covered by optically absorbent material placed on top of it. A disadvantage of this is that the production of such structures is comparatively costly since among other things two additional layers of material for optically blocking off suitable regions of the THz antenna array have to be deposited—this at least involves an electric insulation layer for insulating the electrodes of neighbouring THz antennae and deposited on top of this a layer impermeable to light which usually takes the form of a metal layer. An illustration in cross-section of such a THz antenna array is shown in FIG. 1. The additional optically screening layers identified there may generally adversely affect the performance of the antenna arrangement. It has been shown that the dark current is comparatively high since as a rule more than 50% of the total dark current is generated in the screened regions of the THz antenna array. This results in higher energy consumption by the THz antenna array in the case of a THz emitter or in lower sensitivity in the case of a THz detector. Moreover, the production of such an array has proved to be comparatively costly.
A simplified structure and simplified production of a THz antenna array of the type identified at the outset would be desirable.